The old instance version is 8.10.5 installed using omnibus package. The omnibus package contains Postgres database, which needs to be updated from 9.2.x used in 8.10 release to 9.6.x used in the current Gitlab 10.4.4.

The documentation is not entirely complete, as I be not many people are running Gitlab instance from 2015 that hasn't been updated. There are few mentions of gitlab-ctl pg-upgrade command but the command is not available in Gitlab 8.10.5, trying to run that will provoke gitlab-ctl to give the following response -- "i don't know that command"

What I eventually figured out, and what needs to be done is to update to version 9.5.10 first -- that would update included postgres to 9.6.x